Remove the out-of-date design doc and link. (#2076)

* Remove out-of-date design and links

* Move data_preparation into archived.

Author: brightcoder01

docs/designs/data_preparation.md renamed to docs/designs/archived/data_preparation.md

@@ -37,8 +37,3 @@ cloud, namely, Google Kubernetes Engine (GKE).
 We have also provided [this tutorial](model_building.md) to illustrate how to
 define necessary components that will be used for ElasticDL model training,
 evaluation, and prediction.
-
-## Data Preparation
-
-[This tutorial](data_preparation.md) demonstrates how to prepare datasets into
-the right format that can be used by ElasticDL.